1. Field of the Invention
The present invention relates to an optical disk apparatus, and more specifically to a structure of synchronization information.
2. Description of Related Art
Optical disk apparatuses have hitherto optimized recording power by means of a technique called OPC (Optimum Power Control) and ROPC (Running Optimum Power Control), thereby recording data. According to the OPC technique, prior to data recording, test data is recorded in a predetermined area (PCA) of an optical disk while the recording power is varied in various ways, and the test data is reproduced. Recording power at which the quality (e.g., a β value) of the reproduced signal is optimized is selected as optimum recording power. In the meantime, according to the ROPC technique, consideration is given to the fact that the sensitivity of a recording film is not necessarily uniform within the plane of an optical disk. Then, when data is actually recorded at the optimum recording power determined through OPC, the quantity of return light at the time of data recording is monitored, and the recording power is controlled by feedback such that the quantity of return light assumes a fixed value. In general, the quantity of light of level B is used as the quantity of return light. Here, the level B represents the quantity of return light acquired as a result of formation of a pit when light of recording power is irradiated (i.e., the quantity of return light arising as a result of the recording power having been diffracted by the pit). Specifically, (in a case of an optical disk whose reflectivity is lowered due to data recording) when the quantity of return light of level B is below a given value, it is determined that pits are formed to an excessive degree, thus reducing the recording power. On the contrary, when the quantity of return light of level B exceeds the given value, it is determined that pits are not formed to a sufficient degree, thus increasing the recording power.
Here, data having lengths of 3 T to 11 T (T is a reference clock length) is recorded on a CD-R/RW or the like, and it is difficult to monitor the quantity of return light resulting from light being diffracted by pits having short data lengths and control the recording power through feedback. In view of this difficulty, the quantity of return light at the time of recording of data having the longest pit length of 11 T is detected, to thus effect ROPC. The CD-R/RW complies with specifications in which data of a length of 11 T is included in synchronization information (SYNC) and two data sets, each having a length of 11 T, always appear twice successively in each frame. Accordingly, as one of the two data sets, each having a length of 11 T, always corresponds to a mark (i.e., a data portion forming a pit by irradiation of recording power), ROPC is carried out periodically (i.e., at the timing of periodically-inserted synchronization information) through use of the data having the length of 11 T, thereby allowing periodical optimization of the recording power.
In the case of a DVD-R/RW or the like, however, while data is formed to lengths of 3 T to 11 T as in the case of the CD-R or the like, the DVD-R/RW complies with specifications in which, contrary to those of the CD-R or the like, a data length of 14 T is used for synchronization information and such synchronization information having a 14 T pit length is inserted only once in one frame (93 bytes), not twice successively (i.e. as a pair of a mark and a space). Further, selection of the synchronization information having a data length 14 T as a mark or a space is arbitrary. For example, when all of data having a pit length of 14 T is set to spaces, pits are not formed to the longest pit length of 14 T, and hence ROPC cannot be performed. Consequently, the recording power cannot be periodically optimized, which in turn poses difficulty in maintaining recording quality. It is of course possible to consider that ROPC is enabled by forming all of the data having a 14 T pit length with marks. When all of the data having a 14 T pit length is formed with marks, however, a DSV (Digital Sum Value) may increase, which results in an increase in low-frequency components. Here, the DSV (Digital Sum Value) is a value determined by accumulating all bits in a sequence, which takes two statuses, from the top, while one status (e.g., 1) of the bit sequence is taken as +1 and the other status (e.g., 0) of the same is taken as −1. The smaller the DSV, the smaller the low-frequency components (DC components), resulting in an improvement in recording and reproducing quality.
Moreover, an HD-DVD may suffer from the same problems as described above with regard to DVD-R/RW. Specifically, in HD-DVD, while data is formed to lengths of 2 T to 11 T and a data length of 13 T is used for synchronization information, such synchronization information having a 13 T data length is inserted only once in one synchronous frame.
In view of these circumstances, Japanese Patent Laid-Open Publication No. 2003-91819 describes a technique for setting marks and spaces as synchronization information items such that they are arranged alternately. According to this technique, ROPC, or the like, is periodically performed with respect to the synchronization information items which are marks, thereby allowing optimization of the recording power.
The above technique, however, poses a problem that the DSV is not necessarily suppressed even when the synchronization information is formed such that a mark and a space are arranged alternately. Specifically, the DSV is determined by the synchronization information and subsequent modulated data, and therefore the DSV is minimized in consideration of both the polarity (i.e. mark or space) of the synchronization information and the polarity of the modulated data. In this case, however, if the synchronization information is uniformly set such that a mark and a space are arranged alternately, there arises a necessity for adjusting the DSV only by means of the modulated data, leading to a possibility that the DSV may not be suppressed.
Further, if the synchronization information is uniformly fixed such that a mark and a space are arranged alternately, there arises a portion on a rewritable optical disk, for example, which always corresponds to a mark at the time of recording data, causing deterioration of characteristics of a recording film to further decrease the number of times data on the optical disk can be rewritten.
On the other hand, in a case of forming the synchronization information on an optical disk having a land pre-pit (LPP) formed thereon, such as DVD-R and DVD-RW, if the synchronization information having a data length of 14 T is formed in synchronization with the synchronization LPP, there arises a problem that accuracy of detecting the LPP signal varies depending on the polarity of the synchronization information. In particular, when the synchronization information is formed with a polarity of a mark, the amplitude of the LPP signal decreases to thereby deteriorate the detection accuracy. More specifically, the LPP is formed in synchronization with a wobble signal, and the LPP signal is superimposed on the peaks of the wobble signal. Here, it is possible to extract the LPP signal by means of appropriate setting of a binary threshold value and window setting as long as the LPP signal has a sufficient level. However, when a laser power which is modulated is irradiated onto an optical disk so as to record the synchronization information with a mark having a data length of 14 T at the position of the LPP, heat is dissipated over to an adjacent land in the process of forming the mark of 14 T with the irradiated laser power, which may cause deformation of the LPP in the adjacent land to thereby deteriorate the LPP signal level. Further, in the case of data reproduction after the synchronization information is recorded with a mark having a data length of 14, the following problem occurs. Specifically, while the LPP signal is superimposed on the wobble signal, the LPP signal may be deformed under the influence of recording of the 14 T mark, thereby lowering the signal level. In addition, as the mark portion having a length of 14 T forms a pit, the reflectivity of this portion itself also decreases, which makes it still difficult to extract the LPP signal from the reproduced signal. A similar problem may arise in a case where data is reproduced after data recording. In other words, the synchronization information of 14 T and the LPP are present at a synchronized position, which means that the LPP signal is included in the 14 T portion in a reproduced signal, as a result of which an error may be caused in detecting the 14 T synchronization information.
It is therefore desired to set the polarity of synchronization information such that accuracy of detecting an LPP signal is increased and ROPC can be secured with a mark appearing at a desired frequency, and the DSV can also be suppressed.